Design Guide 139
I/O Controller Hub
9.7.2.5 General Power and Ground Plane Consideration
To properly implement the common mode choke functionality of the magnetics module, the chassis
or output ground (secondary side of transformer) should be separated from the digital or input
ground (primary side) by a physical separation of 100 mils minimum.
Good grounding requires minimizing inductance levels in the interconnections and keeping ground
returns short, signal loop areas small, and power inputs bypassed to signal return. These will
significantly reduce EMI radiation.
The following are guidelines that help reduce circuit inductance in both backplanes and
motherboards:
• Route traces over a continuous plane with no interruptions (don't route over a plane split). If
vacant areas exist on a ground or power plane, avoid routing signals over the vacant area.
Routing over a vacant area will increase inductance and EMI radiation levels.
• Separate noisy digital grounds from analog grounds to reduce coupling. Noisy digital grounds
may affect sensitive DC subsystems.
• All ground vias should be connected to every ground plane, and every power via should be
connected to all power planes at equal potential. This helps reduce circuit inductance.
• Physically locate grounds between a signal path and its return. This will minimize the loop
area.
• Avoid fast rise/fall times as much as possible. Signals with fast rise and fall times contain
many high frequency harmonics, which can radiate EMI.
• The ground plane beneath the filter/transformer module should be split. The RJ45 connector
side of the transformer module should have chassis ground beneath it. By splitting ground
planes beneath transformer, noise coupling between the primary and secondary sides of the
transformer and between the adjacent coils in the transformer is minimized. There should not
be a power plane under the magnetics module.
Figure 9-18. Ground Plane Separation
Separate Chassis
Ground Plane
Magnetics Module
Ground Plane
0.10" minimum Separation
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